After 5 days unsold items are supposed to be returned to the seller. That does not work and the item get a negative number of days making the buy option unusable
When your Sim goes on vacation all stuff that he gave to the consignment store are going back into the Sims inventory and instead of emptying out the list in the Store, the list gets deleted rendering the register unusable.
Cancel all items with a negative number of days left.
Bulldozing/ building a new store might help. To omit, get all your consignments back before going on vacation.